BRITISH FOSSILS. 



Decade VI. Plate I. 



^- .. ELASMODUS HUNTERI. 



[Genus ELA,SMODUS {ixaa^a, a plate, and bUhs, a tooth). Egerton. (Sub-kingdom 

 Vertebrata. Class Pisces. Sub-class Elasmobranchii. Bonaparte. Order Holocephali. 

 MijLLEE. Family EdaphodontidjE. Owen.) Lower maxillaries having one tritor on each 

 structure composite, partly of bone, partly of laminated dentine ; upper maxillaries bony, 

 intersected by a scroll of laminated dentine forming the outer margins, and a double 

 tritoral tooth on each ; premaxillaries composed wholly of laminated dentine.] 



Synonyms. — Chimcera Ilunteri, Owen, Odontography, (1840-45), p. 66 

 Elasmodus Hunteri, Egerton, Proceedings of Geol. Soc, 1843. 



Description. — The dental apparatus of Elasmodus is so compli- 

 cated in its structure, that it is a task of some difficulty to com- 

 municate to the reader an exact idea of its many peculiarities. 

 However adroitly this may be accomplished, an inspection of the 

 clear and faithful representations given in the plate will convey a 

 more accurate notion of these curious fossils than any verbal descrip- 

 tion. The arrangement of the several tissues constituting the lower 

 jaw is as follows : — The anterior portion, comprising nearly one 

 third (figs. 1, 2, a.), is composed of a series of laminse of coarse 

 dentine, superimposed on one another in the following manner. At 

 the symphisis the plates incline downwards and outwards, while on 

 the external front they maintain an horizontal direction for one 

 half their extent, and then slope downwards and backwards to their 

 termination. These are succeeded by a vertical band of coarse 

 fibrous bone (figs. 1, 2, h) occupying the middle third of the jaw. 

 The posterior third (figs. 1, 2, c.) has its outer wall composed of a 

 series of vertical columns of laminated dentine, the points of which 

 give to this part of the cutting edge a notched and irregular outline 

 as in the recent Chimcera monstrosa. Between this columnar portion 

 and the bony band a thin line of dentine (figs. 1, 2, d.) occurs, formed 

 by the continuation of the inner edge of the large triturating tubercle, 

 which traverses the entire substance of the bone. The tubercle itself 

 is composed of dentine, and occupies a considerable part of the 

 [VL L] 6 B 



